The invention relates to a solar cell with p- or n-type base. The solar cell comprises a semiconductor substrate, e.g. silicon, which is covered on a back side by the p- or n type back surface field (BSF) layer and on a front side by an emitter layer of a conductivity type opposite to that of the base layer. The front side of the substrate is arranged to be directed towards a light source during use of the solar cell.
Document WO 2009/064183 discloses a process for creating of a n-type BSF layer and a p-type emitter in a silicon substrate comprising: providing a crystalline substrate having a first side and a second side opposite the first side, pre-diffusing Phosphorus into said first side of the substrate, blocking said first side of said substrate, diffusing Boron into said second side of said substrate and simultaneously diffuse said Phosphorus further into the substrate. That document further discloses that the first side can be blocked using the first side of another substrate. Thus, that document discloses that it is possible to form a solar cell structure by diffusing Boron onto substrates with a pre-existing BSF layer if the BSF layer is protected, for example, by placing the back sides of two substrates against each other (‘back-to-back’ loading).
Document WO 2011/025371 discloses a process for creating a p- or n-type BSF layer and an n- or p-type emitter in a silicon substrate. This process comprises the step of diffusing a dopant on both the textured front side and the back side of the silicon substrate. Subsequently, the resulting glassy layers are removed from both sides, and the doped layer on the front side is removed and replaced in a diffusion process through a doped layer of the opposite conductivity type. The gist of this process is that a co-diffusion of the emitter on the frontside and the BSF on the backside is feasible in combination with a non-single sided diffusion to pre-diffuse the BSF layer. The requirement thereto is that the frontside is etched before the co-diffusion, which can be done in a way that retains the existing surface texture. The two-sided diffusion of the BSF layer may be carried out in a variety of doping methods, including the use of a belt furnace and diffusion sources applied by spray, vapour, spinning, printing or plasma implantation doping.
A disadvantage of the known manufacturing process is that an emitter with a high sheet resistance will have a lower junction depth, which brings the junction closer to a front side metal contact which may increase recombination losses and reduces cell efficiency.
A further disadvantage of the known manufacturing process is that removing the first conductivity-type doped layer from the textured front surface by an etching process adapted for retaining texture of the textured front surface is a critical process. Over-etching on the front side leads to enhanced optical losses due to the loss of the surface texture. Under-etching would lead to shunting problems because the remaining first conductivity-type doped layer can compensate the subsequently diffused second conductivity-type doped layer. The critical nature of the front side etching process may cause some yield losses in high-volume solar cell production.